Escape from Prisoner's Dilemma in RNA phage phi6

We previously examined competitive interactions among viruses by allowing the RNA phage phi6 to evolve at high and low multiplicities of infection (ratio of infecting viruses to bacterial cells). Derived high-multiplicity phages were competitively advantaged relative to their ancestors during coinfection, but their fixation caused population fitness to decline. These data conform to the evolution of lowered fitness in a population of defectors, as expected from the Prisoner's Dilemma of game theory. However, the generality of this result is unknown; the evolution of viruses at other multiplicities may alter the fitness payoffs associated with conflicting strategies of cooperation and defection. Here we examine the change in matrix variables by propagating the ancestor under strictly clonal conditions, allowing cooperation the chance to evolve. In competitions involving derived cooperators and their selfish counterparts, our data reveal a new outcome where the two strategies are predicted to coexist in a mixed polymorphism. Thus, we demonstrate that the payoff matrix is not a constant in phi6. Rather, clonal selection allows viruses the opportunity to escape the Prisoner's Dilemma. We discuss mechanisms that may afford selfish genotypes an advantage during intrahost competition and the relevance in our system for alternative ecological interactions among viruses.